Aggregation

Introduction

Aggregation is a specialized form of association that represents a “has-a” relationship where one class contains a collection of other classes, but the contained classes can exist independently of the container.

Think of it as a weak ownership — the parent holds a reference to the child, but the child can exist on its own.

Real-World Analogy: Department and Teachers

A Department contains Teachers:

If the department is closed, the teachers still exist
Teachers can work in multiple departments
Teachers have their own identity (employee ID)

Characteristics

Characteristic	Description
Relationship	”Has-a” (weak ownership)
Lifetime	Parent and child have independent lifetimes
Sharing	Child can be shared across multiple parents
Destroy parent	Child objects continue to exist
UML	Empty diamond (◊)

Example

class Player {
  constructor(name, skill) {
    this.name = name;
    this.skill = skill;
  }
}

class Team {
  constructor(name) {
    this.name = name;
    this.players = [];
  }
  addPlayer(player) {
    this.players.push(player); // Aggregation
  }
}

// Players exist independently
const messi = new Player("Messi", 99);
const ronaldo = new Player("Ronaldo", 98);

// Players are added to a team
const dreamTeam = new Team("Dream Team");
dreamTeam.addPlayer(messi);
dreamTeam.addPlayer(ronaldo);

// But players still exist even without the team!
console.log(messi.name); // Works!

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